1. Technical Field
The present invention relates to medical implantable devices and more particularly to the test of medical lead impedance of implantable neurostimulators used for paraplegic people.
2. Background Art
It is a reality that either for natural reasons or consecutively to a malady or a traumatism, nerves or muscles of the human body may be affected and suffer from lack of stimulation or at least from a deficient stimulation. Electronic solutions to replace such deficiencies use Functional Electrical Stimulation devices (FES). State of the art of such devices mainly address the crucial problem of the heart attack with the well-known pacemakers. However, another important concern in which research of implantable solution is very active is the paraplegia domain in which the muscles are inert due to spinal injury.
Contrary to some devices like pacemakers which run continuously and need real time measurements, the implantable devices for paraplegic alternate active and inactive time periods which offer room for different measurements. Furthermore, as the generation of human walking is a complex algorithm, a great number of medical leads are used at the same time to activate the muscles of the inactivated legs (typically 16 or more leads are used as compared to two leads for cardiac pacemaker). In case some of the leads break, the device may be still active, and it is less vital that a real time integrity measurement be made for paraplegia devices than for pacemakers. Therefore, implantable devices for paraplegic do not need to be tested in real time, and checking can be done either at the system setup or during an active stimulation.
Generally, an implantable device performs two main functions: stimulation and telemetry. The stimulation is obtained by the generation of electrical pulses in order to deliver a current through a network of electrodes and medical leads in contact with the part of the body to be stimulated (muscle or nerve). The telemetry function is a feedback operation which allows to get information onto the integrity of the implantable device, such as measuring the level of power supply of the component or testing the value of the lead impedance formed by the association of the stimulated tissue plus the electrode in contact with it plus the wire connecting the electrode to the implantable device.
A first problem arises when the network of electrodes is placed within the body of a patient because several damages may affect the integrity of the leads such as partial or a total fracture in the electrical wires. However, the leads may be tested during the surgical operation which is not the purpose of the present invention.
Moreover, such kind of injury and others such as fibrosis may also appear during the time period the patient is using the stimulation system. It is therefore mandatory that these elements be tested regularly to be sure that stimulation pulses are sent with efficiency. Because of the non-accessibility of the F.E.S. device which is embedded within the patient body, the testing of the components is not easy. However, the testing of lead impedance has been addressed in many patents and only a few are described immediately hereinafter.
U.S. Pat. No. 4,949,720 discloses an apparatus for measuring the lead impedance in a pacemaker. The invention includes a large number of FET transistors operated in parallel to discharge a capacitor though the heart tissue. Pacer lead current is monitored by measuring the current through a small number of these transistors. The current monitoring function is performed by a current-to-voltage converter coupled to an analog-to-digital converter which may make one or more voltage measurements during the output pulse.
U.S. Pat. No. 4,140,131 disclose an apparatus for stimulating body tissue and in particular the heart, as including a device or circuit responsive to the initiation of stimulation and/or to the failure or pending failure of a component of the stimulating apparatus to provide the patient with a perceivable stimulation to a second, different portion of body tissue. There is disclosed an impedance level detector for sensing the impedance presented between the outputs of the stimulation apparatus to provide a warning signal indicating that the output impedance falls outside a predetermined range. In particular, the impedance level detector output is sensed by stimulation control logic to apply a first train of pulses at a first rate to an auxiliary electrode for stimulating the second portion of tissue. Further, there is included a voltage level detector for sensing when the power source voltage depletes below a predetermined level, to actuate the stimulation control logic to provide a second train of warning pulses to the auxiliary electrode, at a second, different rate than that of the first train. In this fashion, the patient not only is warned as to the pending failure or failure of a component of his pacemaker, but also is able to identify the failing component.
With these solutions, either the voltage (V) or the current (I) are measured across the lead impedance (R) and the final value of the impedance is obtained by computing the well-known Ohm""s equation: V=Rxc3x97I.
Another approach to determine the integrity of the leads consists in measuring a stimulation capacitor voltage and to deduce the impedance lead value from a well-known general equation:
V=V0xc3x97e(xe2x88x92T/RC),
wherein V0 is the initial voltage of the stimulation capacitor and (1/RC) is the time constant. Such method is illustrated in the two following patents: In U.S. Pat. No. 5,891,179 from Er et al., a real-time impedance monitoring system is provided for use with an implantable medical device having an implantable electrical lead. The impedance monitoring system includes components for determining the electrical impedance of the lead as a function of time, with the determination being made substantially in real-time, and components for graphically displaying the electrical impedance of the lead as a function of time, with the display also being generated substantially in real-time.
In U.S. Pat. No. 5,201,865 from Kuehn, a method and apparatus for measuring lead impedance during pre selected test mode operation of an implantable body tissue stimulator is presented. Analysis circuitry is periodically triggered into operation, such as on each reprogramming by the physician or periodically as a function of elapsed time or number of stimulation events counted from the preceding measurement. The actual lead impedance, measured from the output circuit from the body tissue-stimulator pulse generator, and taking into account impedance of the interconnection between the lead connector pin and the pulse generator connector block, the lead electrical conductor and its connections with the electrode and the connector pin and the electrode-tissue interface, is calculated as a function of the ratio of the elapsed times that it takes to discharge a capacitor from a first reference voltage to a second reference voltage through a precision resistor and through the lead impedance itself. The calculated lead impedance may be stored in memory with a suitable time tag, employed to automatically effect a change in operating modes or change a lead and electrode selection, if measured lead impedance falls outside normal high and low impedance boundary values. In the pacing context, calculated lead impedance may be employed to adjust sense amplifier sensitivity and pacing output pulse parameters. The method and apparatus may also be employed to calculate cardioversion/defibrillation lead impedance through selective partial discharge of high voltage output capacitors.
The ""179 solution implies that the voltage measurement be picked at the end of the impedance path and thus that an extra-wire from the implant is required. Then, in case of paraplegia where a great number of electrodes are required, such solution is not acceptable due to the number of extra-wires that would be required.
Furthermore, in paraplegic application, the leads are not necessarily proximate to the FES device and the wiring may be long and difficult to access, thereby rejecting a solution as the aforementioned one.
In the ""865 patent, the reader may assume that all the circuitry is powered with a unique common voltage VDD. In the case a high power voltage (i.e. 40 volts) is used for the stimulation capacitor and a lower voltage (5 volts) is used for the rest of the circuitry, this prior art solution is not acceptable as such because the circuitry must be adapted.
An important constraint of paraplegia is the need of interconnection of low powered components (the implant is powered less than 5 v) with high power output stages (high voltage over 40V and high current over 20 mA ) which are needed for the stimulations.
Therefore, even if the cited pacemakers solutions are convenient for the measurement of lead impedance in the cardiology context, none of the aforementioned patents offers a complete solution for testing the functionality of leads impedance in paraplegic implantable device.
Thus, there is still a need for a simple, short time functional testing solution for paraplegic implantable stimulator.
It is therefore a feature of the present invention to provide an apparatus for testing the impedance of a medical lead connecting an implantable stimulation device to a nerve or a muscle. The implantable device is of the type comprising a capacitor for stimulating the nerve or the muscle. The system of the invention comprises a current generator for generating a testing current xe2x80x9cIxe2x80x9d during a calibrated testing pulse and a power circuit coupled to the capacitor and to the current generator for determining if the capacitor is charged by the testing current during the calibrated testing pulse.
Various other objects, features, and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views.